How much do biometric devices cost? And what are the size limitations?

Putting a thumbprint reader at every cash register doesn't make sense, but putting one in the card does. The financial institutions are the entities making the money from merchant fees, holding the liability for fraud, and have a vested interest in catering to customers that use their services (like a credit card) a lot.

What if, say, platinum customers only (or platinum plus usage... you get a special card after you spend your first $5K or $10K) received an expensive card that was capable of rudimentary biometric readings? Then the auth system could verify the numbers generated from your thumbprint as well, reducing the frequency of annoying and embarrassing declines for the highest value customers.

I bet visa, mastercard, and amex would swallow up a startup that prototyped the technology for a pretty penny... even if they didn't use the thing for another half decade. Not to mention the half dozen super-banks (BancAmerica, Citigroup, etc) and established security tech companies.